refactor: make mod metadata presence (or lack of) easier to find out

launcher/minecraft/mod/ModDetails.h

@@ -7,6 +7,12 @@
 
 #include "minecraft/mod/MetadataHandler.h"
 
+enum class ModStatus {
+    Installed,      // Both JAR and Metadata are present
+    NotInstalled,   // Only the Metadata is present
+    NoMetadata,     // Only the JAR is present
+};
+
 struct ModDetails
 {
     /* Mod ID as defined in the ModLoader-specific metadata */
@@ -30,6 +36,9 @@ struct ModDetails
     /* List of the author's names */
     QStringList authors;
 
+    /* Installation status of the mod */
+    ModStatus status;
+
     /* Metadata information, if any */
     std::shared_ptr<Metadata::ModStruct> metadata;
 };